X-ray single crystal diffraction reveals that a series of cocrystals are assembled by three ring angular diazaphenanthrenes including 1,7-phenanthroline, 4,7-phenanthroline and 1,10-phenanthroline with 1,4-/1,2-diiodotetrafluorobenzenes via C-I···N halogen bonding (XB) as main driving force. Raman shift of the symmetric CI stretching vibration coupling with ring elongation and lateral ring expansion to a lower frequency by 2 to 7cm-1 for 1,4-DITFB in cocrystals shows the existence of C-I···N halogen bonding. All cocrystals phosphoresce with a distinct change of colors from yellow, orange, pink to red. Also phosphorescent lifetimes of cocrystals containing 1,4-DITFB are longer than those of others constructed by 1,2-DITFB. These phenanthrolines monomers have almost same phosphorescence wavelength (the max. at 493nm) in β-cyclodextrin solution in the presence of bromocyclohexane as a pure physically heavy atom perturber. The results demonstrate CI···N XB makes heavy atom effect more direct and efficient, and influences significantly the energy level of the lowest lying excited triplet states and the population of electrons to triplet state of the angular diazaphenanthrenes because of greater contribution of lone pair electrons from nitrogen to conjugation systems. Meanwhile, the XB modulates luminescent behaviors due to difference in positions of nitrogen atoms. Good coplanarity, i.e., torsion angles being closer to 0°, in CI···N halogen bonded binary systems also is an important factor affecting the appearance quality of cocrystals.
Keywords: CI···N halogen bond; Cocrystals; Coplanarity; Phosphorescence.
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